Sour Water Strippers Exposed

7/28/2019 Sour Water Strippers Exposed

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SOUR WATER STRIPPERS EXPOSED1

Ralph H. WeilandOptimized Gas Treating, Inc.

P.O. Box 125

Clarita, OK [email protected]

Nathan A. HatcherOptimized Gas Treating, Inc.

311 Bayou Bend

Buda, TX [email protected]

512-970-5019

Abstract

Sour water strippers (SWS) are moderately-large reboiled or live-steam-injectedtowers (30 to 60 trays) in which ammonia and other gases are removed from the sour water

by steam stripping. Heretofore they have been designed using equilibrium stages. However,

tray efficiencies have remained obscure with quoted values anywhere from 15% to 45%, afactor of three range. Designers therefore have less than complete confidence in the

reliability of their final design. The consequence of uncertainty is either overdesign and,

therefore, excessive costs or sleepless nights because of an underperforming unit.

Recently a mass transfer rate-based simulation model has become available fordesigning and troubleshooting sour water strippers. In this paper, we use the model to

determine tray efficiencies for ammonia and H2S stripping, how they vary across the height

of a tower, and what operating variables affect them. We also predict quantitatively how thepresence of heat stable salts affects the treat-ability of sour water to specified residual levels

of ammonia and H2S.

1PaperpresentedattheLaurenceReidGasConditioningConference,Norman,Oklahoma,February28,2012
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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removal, TGU quench columns, and various effluent drains for removing the water used to

prevent salt deposition in equipment (Stevens et al., 2007).

Finally, it may be useful to point out that ammonia and hydrogen sulfide have almostunlimited solubility in water when they are present together. This is an interesting

consequence of the fact that the reactive component of the solvent, ammonia, is volatile and,

if present in the gas phase, it will continue to absorb as long as it becomes protonated as a

result of H2S co-absorption. Thus, it is conceivable that a particular sour water stream may bea lot more concentrated than the solubility of ammonia itself would suggest.

Basic Stripping Process

Sour water stripping is a fairly simple process in which either external steam, steam

generated by a reboiler, or even a hot hydrocarbon stripping vapor is used to shift chemicalreaction equilibria by heating the sour water. The steam is the gaseous solvent used to

remove and carry the ammonia and H2S out of the system. In other words, the application of

heat generates internal stripping steam (equivalently, live steam injection can be used) and

carries ammonia, H2S, and CO2 out of the water by:

1. Heating the sour water feed to boiling point2. Reversing chemical reactions3. Diluting the partial pressure of the gases stripped by furnishing excess steam

This sounds a lot like an amine regenerator, and it is. Figure 1 shows a typical SWS columnwith heating by the injection of live steam. Because a sour water stripper does not form a

fully

Figure 1 Sour Water Stripper with Live Steam Injection

closed loop like an amine regenerator does, maintaining a water balance is unnecessary. This

means that live steam can be used as a stripping agent either alone or in conjunction with aconventional reboiler and the additional water added by the condensate simply added to the

refinerys water inventory. Typical energy usage in the stripping process is in the range 1.0

1.5 lb of 50 psig equivalent saturated steam per gallon of sour water.


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When an external reboiler is used, steam pressure is often higher than in an amine

regenerator to minimize heat exchange surface. In an amine regenerator, amine degradation

limits temperatures. In a sour water stripper, ammonia recycle in the stripped sour water isundesirable to begin with, so these concerns do not exist. However, there is a practical limit

of 400-450F where coking heavy hydrocarbons can lead to fouling and solids deposition in

the reboiler, and corrosion is always a concern.

Higher NH3 and H2S concentrations require more stripping energy, but a higherconcentration is also a more efficient way to store and transport the NH3 and H2S removed

from upstream units. Because high H2S solubility relies on the presence of ammonia, the

molar concentration of H2S very rarely exceeds that of ammonia, and then usually only indilute systems. A typical molar ratio of H2S to ammonia is 0.5-0.8 in the combined SWS feed

water of a typical refinery. Ammonia levels in the water are often determined by upstream

process conditions, and they can be highly specific to the process licensor and crude slate inoperation. Obviously, higher concentrations of NH3 and H2S are preferred from a water

consumption perspective. However, there is a practical limit of between one and several

weight percent ammonium bisulfide equivalent in the sour water feed before metallurgy mustbe significantly upgraded.

Trays have historically been used in SWSs, but random packing is beginning to see

use in units processing relatively clean water. Trays with directional, fixed valves have beenreported to be more resistant to fouling because the horizontal velocity imparted as the gasleaves each valve tends to sweep clean the area near the valves (Hauer and Kirkey, 2003).

Stripped sour water specifications for NH3 and H2S can be highly dependent upon the

locale where the unit is installed and the final discharge requirements. NH3 is harder to stripthan H2S and typical targets for NH3 are 30-80 ppmw in the stripped water versus

undetectable to less than 0.1 ppmw for H2S. Typical recent installations (Quinlan and Hati,

2010; Asquith and Moore, 2000) involve 35-45 actual trays with tray efficiencies quotedanywhere from 25 to 45%.

In some cases, other alkaline contaminants besides ammonia may be present in sourwater. Amine can carry over into the regenerator purge or it can be present from injection

into the crude unit overhead for corrosion control. Sodium, potassium, and magnesium mayalso be present from impurities in the makeup water (hardness) or by water-contactingvarious products containing these compounds within the upstream units. These species can

chemically trap additional H2S and prevent the H2S from being stripped. In order to spring

the H2S, acid then must be added to the water. Other contaminants and their effect will be

deferred to the next section on the chemistry of ammonia-acid gas systems.

Chemistry of Ammonia with Acid Gases

Ammonia is a relatively weak base capable of being mono-protonated. For example,in aqueous solution it forms ammoni otonated ammonia) to a limited extent:um ion (pr

NH3 HNH4

(1)

This reaction is in perfect parallel with amine protonation, so in this sense ammonia can be

thought of as nothing more than just another reactive amine. There is a great deal of nonsense

written in various books and other publications concerning the reactions of H 2S and CO2 with

ammonia. With the exception of carbamate formation by the direct reaction of CO2 withNH3, all other reactions involve only ionic species. Ions do not form ionic compounds except

possibly when materials begin to precipitate. As long as all species are in solution, they exist


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as ions and in no sense are they associated with each other, forming compounds. All ions are

freely floating and the only associations are ones that result in solution nonideality. Thus,

there is no such thing as ammonium carbonate, ammonium bicarbonate, ammonium bisulfideor diammonium sulfide. Such compounds are pure fictions, and it is very hard to make sense

of reaction equilibria unless one discards such notions.

The reactions that occur when H2S and CO2 dissolve in solution are the same as in

any other aqueous, primary or n tem. The reactions are:seco dary amine sys

(2)

(3)

(4)

(5)

(7)

(6)

Ammonia influences all these reactions by serving as a proton sink per reaction (1).

Understanding sour water strippers, understanding the way ammonia behaves as a solvent foracid gases, and understanding what happens to ammonia in amine regenerators depends on

knowledge of

vapor-liquid equilibria, kinetics of the carbamate forming reaction, Equation (7), and an appreciation for mass transfer as a rate process where Henrys law applies.

The thermodynamic framework on which the vapor-liquid calculations are based

involves a model for the activity coefficients via extended Debye-Hckel theory. Thus, thebasic modeling approach for ammonia is the same as for any other amine with one exception:

Ammonia is highly volatile and this leads to different distributions in absorption and

especially regeneration equipment compared with amines. Its volatility is responsible for it

getting into the sour water system (through condenser water blow down, for example) but

volatility also allows it to be recovered relatively easily from the same sour water. Heatstable salts and alkali salt contaminants also affect the distribution of ionic species in solution

and their presence must be considered, too.It is very common in refinery cracking units (FCCs and Cokers) for the sour water

generated to contain organic and inorganic acid impurities from heat stable salt precursors.

Just as for amine units, ammonia will be converted into the protonated form. It cannot bethermally regenerated because the HSS responsible for the protonation is completely

nonvolatile and cannot be removed by boiling it into the stripping steam. In these cases, it isquite common to inject a small amount of strong base (NaOH) to shift the pH into a range

where ammonium ion, NH4+, shifts back to NH3. Spent caustic from Merox-type units is

commonly used for this purpose, but care must be taken to ensure that disposal of the spent

caustic is not completely reliant on this destination, or the tail will begin to wag the dog.When adjusting the pH of the water to spring ammonia chemically, the adjustment is

usually made by metered injection of caustic onto a tray far enough down the column that

most of the H2S has already been stripped out and ammonia is the main remainingcomponent. The metering rate is controlled to a set-point on the pH measurement in the

stripped water after it has been cooled. Caustic injection on a lower tray generally works

better than injection directly into the SWS feed itself because the H2S concentration isalready small on lower trays. However, pH is extremely responsive to caustic addition, so


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the measuring and control elements should be as close together in time as possible if rather

large fluctuations in pH are to be avoided. It is important to avoid injecting any more caustic

than absolutely necessary because excess caustic injection permanently binds H2S into thesolution and eventually this will find its way into biological treatment ponds, either reducing

the efficacy of the microbial population, or unnecessarily increasing the biological oxygen

demand.

Simulating Sour Water Strippers

Traditionally, SWSs have been modeled as a series of equilibrium stages with stageefficiencies quoted anywhere in the range from 15% to 45%, i.e., ranging over a factorof

three. However, the mass transfer rate-based approach to the simulation of amine contactors

and such hard-to-model distillation processes as extractive, azeotropic and reactivedistillation have been in commercial use since the mid to late 1980s. Their extension to sour

water stripping is a natural progression and, in December, 2011 a commercial mass transfer

rate-based sour water stripper model became available as part of the ProTreat simulationpackage. The remainder of this paper uses the ProTreat simulators SWS model to reveal

some rather surprising facts and behaviors of sour water strippers.

Phase EquilibriumThe vapor pressure of ammonia and acid gases above sour water plays a key (but by

no means the only) role in determining the extent to which a given column and processconfiguration will purify sour water and how the process operating conditions affect the

separation. We have a developed an activity coefficient model for phase behavior for sour

water that uses the Deshmukh-Mather (1981) approach for activity coefficients, and thePeng-Robinson equation of state for the vapor. The thermodynamic model has been fitted to

the data of Rumpf et al. (1999), Wilson (1990), van Krevelen et al. (1949), Badger and Silver

(1938), Carroll and Mather (1989); Carroll et al. (1991), Otsaka et al. (1985), Goppert

(1985), Muller et al. (1987), Miles and Wilson (1975), Cardon and Wilson (1978), Gillespieet al. (1985), Clifford and Hunter (1933), Butler (1998), Kohl and Riesenfeld (1985),

Edwards et al. (1978), and Beychok (1967), a total of some 550 separate experiments, mostinvolving ammonia and either one or both of the acid gases H2S and CO2. Figure 2 is a set of

parity plots indicating partial pressure ranges and the goodness of fit.

Figure 2 Parity Plots of VLE Data for the system NH3-H2S-CO2-H2O


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Mass Transfer Rate ModelThe sour water stripper model in ProTreat deals with the separation of ammonia, the

acid gases, and water as governed by the particular mass transfer rate of each individualcomponent. Ammonia is treated as reactive towards CO2 in the same sense that any other

primary or secondary amine is reactive and transfer rates are determined by individual (vapor

and liquid) phase mass transfer coefficients, interfacial areas, and concentration difference

driving forces. The details of mass transfer rate modeling have been described at lengthelsewhere (Weiland et al., 2003; Weiland and Dingman, 2001) and will not be repeated here.

Suffice it to say the kinetics of the NH3-CO2 reaction (Eq. 7) have been properly accounted

for using known kinetic parameters and the usual enhancement factor approach. Theresulting process model is a virtual plant on a computer in which all the fundamental physical

and chemical processes occurring are properly accounted for. The computer simulation is a

direct one-for-one prediction of both column and process performance completely withoutrecourse to user estimates of any parameters whatsoever. All that is required is the actual

process flow sheet configuration, tower internals details, and values for process parameters

corresponding to such operating conditions as sour water flow and composition, towerpressures and imposed heat loads.

The SWS model itself has been validated against at least a half-dozen sets of stripper

performance data by two refining companies and an engineering firm, with what werereported by them as excellent results. Using a virtual plant in the form of a precise computermodel, the remainder of this paper examines the effect of steam to water ratio, the presence

of heat stable salts, and stripper operating pressure on SWS performance as measured by

such parameters as Murphree tray efficiency and residual ammonia and H2S in the strippedwater. Consideration also is given to how caustic injection springs ammonia, and the fact

that one can simulate a little more complex water stripping scheme such as the Chevron

WWT process. The bulk of the simulation cases have used live steam injection as the sourceof stripping vapor because this tends to be the most effective approach to providing thermal

energy for stripping; however, reboiled strippers add less makeup water, and they can besimulated just as easily.

Exploring Strippers

Figure 1 shows the simplest possible configuration of a sour water stripper. For this

exercise, the stripper contained 40 one-pass valve trays on 2-ft spacing with 2-inch weirs.

Sour water was fed to Tray 6 (from the top), live steam saturated at 50 psig entered below thebottom tray, and in all cases the column was sized for 70% of jet and downcomer flood.

Except for the case study involving stripper pressure where a fixed temperature approach of

20F was used, the sour water feed to the stripper was kept at 235F. Table 1 gives theconditions of the sour water used for this case study.

Ratio of Steam to Sour WaterFour steam rates were used: 0.78; 0.92; 1.20 and 1.48 lb/USgal, covering the range

normally used in sour water stripping2. One of the performance parameters of interest is the

2 Gantz (1975) shows performance data for steam rates from 0.6 to 4.0 lb/USgal although, commercially, rates

at and above 1 lb/USgal are more usual.


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computed tray efficiency. For H2S, Figure 3 shows the Murphree (1925) vapor-phase

efficiency, ,, defined (Taylor and ooi an, for component i on tray n (from the top) as:K jm

,

,,

, ,

(8)

It should be noted that this is notan overall tray efficiency. Overall efficiencies are based on

equilibrium stage calculations to determine the number of theoretical stages. ProTreat

studiously avoids such calculations because when combined with the averaging that is done

through overall efficiencies, they completely obscure what is really going on in SWSs. The

Murphree vapor efficiency provides a quantitative measure of how close to equilibrium eachreal tray operates. There is no relationship between Murphree and overall tray efficiencies.

Table 1 Sour Water Feed Conditions

Total flow (lb/h) 150,000

Temperature (F) 135

Pressure (psia) 70

Water (mol%) 96.4

Hydrogen sulfide (mol%) 1.5Carbon dioxide (mol%) 1.1

Ammonia (mol%) 2.0

Formate (ppmw) 200

Thiocyanate (ppmw) 100

Figure 3 Calculated Murphree Vapor Tray Efficiencies for H2S

As can be seen from Figure 3, H2S efficiency various enormously throughout the

tower and is a very sensitive function of the steam-to-sour-water ratio. Generalizing H2Scomponent efficiency is utterly impossible because it varies from a few percent to 50%.


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Obviously the tray count and the steam required depend heavily on the treated-water

specifications to be met. Thus, the designer is faced with a difficult decision because more

trays and more stripping energy lead to lower efficiency! These effects cannot be modeledor accounted for with an equilibrium stage model, even when an efficiency is used.

Providing optimal, cost-effective designs using theoretical stages is a rather seat-of-the-pants

enterprise.

Before tackling corresponding efficiencies for ammonia, it is revealing to look at thevapor- and liquid-phase concentration profiles of ammonia, as well as the simulatedvapor

profile compared directly with the equilibrium vapor composition. Figure 4 shows tray-by-

tray profiles of the liquid and vapor phase mole percentages of ammonia. The surprisingthing is the bulge, or maximum, displayed at boil-up rates at the low end of the range.

Figure 4 Ammonia Concentration Profiles in the SWS

We have observed and reported the existence of similar maxima in ammonia

concentration in the context of ammonia control in refinery amine units (Hatcher andWeiland, 2012). Indeed, there it was found that in an amine regenerator, only the bottom fewtrays were effective in stripping ammonia. In the present case of a SWS, ammonia stripped

in the lower part of the column is actually reabsorbed in the upper part because the H2S is

high enough there to react with and fix a significant part of the ammonia back into the liquid.When the steam to sour water ratio is high enough, however, the H2S concentration is

everywhere too low to remove enough ammonia to result in a maximum in the ammonia

concentration. We note in passing that there is a strictly monotonic decrease in H2Sconcentration as the sour water moves down the column.

When the boilup rate is at the lower end of the spectrum and a maximum in ammonia

concentration occurs away from the ends of the stripper, the Murphree vapor efficiency for

ammonia exhibits seemingly odd behavior. Figure 5 shows very surprising efficiency trendsfrom tray-to-tray at stripping steam rates of 0.78 and 0.92 lb steam/USgal. At the lowest

steam rate, the apparent efficiency ranges from -100% to +120% and at 0.92 lb/USgal theapparent efficiency reaches +200%. The reason is that the lines corresponding to actual and

equilibrium vapor phase mole fractions cross (as they must do because a bulge implies that

stripping gives way to absorption at some point in the column) and they have a maximum onimmediately adjacent trays rather than on the same tray. Figure 6 shows the actual (y) and

equilibrium (y*) lines for the steam rate of 0.92 lb/USgal from which it can be seen that both


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exhibit a maximum, the lines cross, and they have peak values on different trays. Referring

to Equation (8) (definition of efficiency), it is easy to see that if the numerator or

denominator changes sign, or the concentration difference in the denominator becomes verysmall, apparent efficiencies can become not only large, but negative.

At first glance, much of this odd efficiency behavior may seem a little academic;

however, when a constant efficiency is applied to an equilibrium stage model, the maximum

in ammonia concentration in the interior of the column, well away from the ends, will notberevealed andsimulation results will tend to be quite optimistic in terms of sour water quality

and stripped gas ammonia levels. This may go some way to explaining the wide variations

reported in tray efficiencies (15 to 45%) in this application. It is worth noting that whenstripping steam rates are moderate to high, the efficiency turns out to be between 35 and

40%, with variations of only one or two percentage points across the whole column.

Figure 5 Murphree Vapor Efficiencies for Ammonia at Low Steam Rates

Figure 6 Actual and Equilibrium Ammonia Concentrations

in the Vapor Phase at Low Steam Rates


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Effect of Stripper PressureFrom time to time, one hears it stated that higher stripper pressures favor sour water

stripping so we decided to put this old wives tale to the test. Table 2 shows the effect of

column head pressure in residual ammonia and H2S levels in stripped water for the same

process configuration and sour water composition shown in Figure 1 and Table 1,

respectively. The simulated performance corresponds to a steam rate of 25,000 lb/h of 50psig steam (1.40 lb/USgal) with the cross exchanger set to a temperature approach of 20F to

minimize the effect of temperature differences across the stripping section. The table shows

that contrary to anecdote, higher pressure has a negative (albeit relatively small) effect ofwater quality with respect to ammonia, but can cause an 80-fold increase in residual H 2S

content of the stripped water (890% loss in performance) under these conditions. Higher

pressures do not seem to favor ammonia stripping, and they certainly have a negative effect

on stripped water quality vis vis H2S.

Table 2 Effect of Stripper Pressure on Residual NH3 and H2S levels in Stripped Water

Stripper Head Pressure

(psig)

Treated Water

Ammonia (ppmw) H2S (ppmw)

10 17.2 0.00002

15 21.1 0.00013

20 26.0 0.00058

25 31.5 0.00197

Effect of Heat Stable Salts on Stripped Water Quality

Figure 7 shows the effect of stripping steam rate and heat stable salt level on thesimulated stripped water quality with respect to residual ammonia and H2S content. The

stripper is the same 40 tray unit shown in Figure 1 and the sour water has the composition of

Table 1, with and without the heat stable salts and levels indicated there. It is readilyapparent from the plot on the left that the presence of heat stable salts forces the retention of

a corresponding concentration of ammonia and no matter how much steam is injected into the

stripper there is a residual ammonia level that simply cannot be removed when there are

heat stable salts in the sour water. Heat stable salts have a negative effect on ammoniastripping. However, they have a beneficial effect on H2S removal because they are generally

stronger acids. A two order-of-magnitude reduction in residual H2S is possible with only a

modest amount of HSS present.

The beneficial effect of HSS on H2S removal is at least partially connected to the factthat when caustic is used, it is injected fairly low in the column. By injecting caustic on one

of the bottom few trays, the benefit of the HSS on H2S removal is retained. The ammonia

released from the HSSs by caustic can be adequately removed using only a few trays rightnear the bottom of the column. Although no results are reported here, the mass transfer rate-

based SWS model is perfectly capably of quantitatively predicting the effect of caustic

injection, the caustic strength or injection rate and the injection position. The highly accuratepH prediction also provides a way to assess reliably the efficacy of pH control.


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Figure 7 Effect of Heat Stable Salts on Stripped Water Quality

SUMMARY

Perhaps the most important message of this paper is that genuine mass transfer rate-

based modeling allows the construction of a virtual plant on a computer. There is no relianceon what might be termed fudge factors to achieve agreement between calculations and

reality.

A mass transfer rate-based sour water stripper model provides a virtual SWS on acomputer. Engineers can now design SWS units with unprecedented accuracy and reliability.

Plant operations personnel can answer a wide range of what-if questions to troubleshoot

operations and to optimize existing units with complete confidence in the results.

Murphree tray efficiencies are probably the most commonly used type of efficiencyfor tray calculations. They are an invention of the 1920s developed to allow ideal stage

distillation calculations to approximate reality more closely, and as 90 year old technology

they have served the distillation business well. However, the whole equilibrium stageapproach was superseded by mass transfer rate calculation methods in the mid to late 1980s,

already nearly 25 years ago. The gas treating industry deals with far more complex

separations processes than distillation but, unfortunately, it has been remarkably slow to

catch on to the new technology. Genuine mass transfer rate-based simulation is an extremelypowerful tool in gas treating.

From this paper there are several other take-home messages that pertain directly to

sour water stripping:

H2S efficiency various enormously from tray to tray throughout a SWS, and it is avery sensitive function of the steam-to-sour-water flow rate ratio. Any attempt togeneralize H2S component efficiency is utterly futile because of its huge variation

from only a few percent to 50% across a column and its dependency on the particular

operating conditions of each unique situation.

With respect to efficiencies pertinent to H2S stripping, the tray count and the steamrequired depend heavily on the treated-water specifications to be met. Thus, the

designer is faced with really quite a difficult decisionmore trays and more stripping

energy lead to lower efficiency! What and where is the balance? Should one usemore or less steam, and more or fewer trays? These effects cannot be accounted for


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with an equilibrium stage model, even when it uses efficiency. The process is too

complex.

At stripping steam rates in the lower half of the normal range used in practice, it isquite possible for the ammonia concentration in a SWS to show a bulge or maximumin some part of the stripping section well away from the ends. At this bulge, the SWS

changes from acting as an ammonia stripper to an ammonia absorber. When (and

where) the bulge occurs, there is a high enough H2S concentration in the liquid toattract ammonia into the water phase and bind it there as ammonium bisulfide.

Again, equilibrium stage models even with efficiency do not show this kind of

behavior, and they can lead to either gross overdesign or to a complete mis-design,

depending on the relative H2S and ammonia concentrations in the sour water feed.

Murphree plate efficiencies for ammonia in SWSs operating at low to moderatestripping steam rates can vary from large negative values to positive values well over

100%. At high stream rates, on the other hand, Murphree vapor efficiencies forammonia stripping are typically 35 to 40%. This huge variation depending on steam

rate and H2S to NH3 ratio may go a long way towards explaining the wide range of

15% to 45% overall efficiency often quoted by practitioners.

Contrary to a commonly repeated anecdote, higher stripper pressures do not favorbetter ammonia stripping, and they certainly do not favor H2S stripping where a

factor of 80 loss in performance was predicted even for the setup in this study.

When there are heat stable salts in the sour water there is a residual ammonia levelthat simply cannot be removed from the water no matter how much steam is injected

into the stripper. HSSs neutralization is needed to reduce the ammonia level further.

Caustic can be used to spring ammonia bound to HSSs. However, it should beinjected as far down the column as possible so as not to interfere with the benefits that

the HSSs have on H2S stripping.

In conclusion, this paper has pointed out several aspects of sour water stripping that

either do not seem to have been recognized heretofore, or that offer solid scientific

explanations for previously observed and reported characteristics.

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Wilson, G. M. and Eng, W. W. Y., GPSWAT GPA Sour Water Equilibria, Research

Report RR-118, Gas Processors Association, Feb 1990.

Sour Water Strippers Exposed

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